1. Field of the Invention
This invention relates in general to magnetic core structures and, more specifically to laminated magnetic transformer core structures having center leg curvilinear S-joints.
2. Description of the Prior Art
Stacked magnetic cores for large electrical power transformers of the core-form type conventionally use the butt-lap type of joint disclosed in U.S. Pat. No. 2,300,964. In the butt-lap joint the ends of the leg and the yoke laminations are mitred and butted together to form diagonal joints between the laminations, in each layer of laminations. In principal, the joints and alternate layers are aligned and offset from aligned joints and the intervening layers. In practice to reduce handling, the joints in three adjacent layers of laminations are usually aligned, and the joints in the next three adjacent layers are aligned, but offset from the joints of the adjacent group of three laminations. While the butt-lap construction can form a good magnetic surface, it has disadvantages. One is the great care with which laminations must be stacked in order to optimize magnetic performance. Another disadvantage is the amount of power loss at the joints which increases the excitation current required and increases the sound level.
Laminated magnetic step-lap transformer cores are also well known in the art. Step-lap joints and transformers reduce core losses, reduce the excitation current requirements, and reduce the sound level, compared with a similarly rated transformer constructed with a butt-lap joint. In a step lap joint, the joints created by the abutting laminations of each layer are successively offset in succeeding layers in the same direction to create at least three "steps" and preferably at least six or seven, before the step pattern is repeated. An example of transformer cores with step-lap joints is disclosed in U.S. Pat. No. 4,200,854. While such prior art cores with step-lap joints improve the magnetic properties of the core, they have left something to be desired in the way of minimizing scrap saving in their construction.
Accordingly, it would be desirable and it is an object of this invention, to provide a laminated magnetic step-lap transformer core with center leg curvilinear S-joints which may be fabricated from less material than present state of the art cores. It is a further object of this invention to improve the no-load performance of a transformer, to lower the cost and to increase the productivity of core manufacturing.